Organic chemicals in the form of nonaqueous-phase (immiscible) organic liquids (NAPLs) occur in the subsurface at numerous contaminated sites and act as long-term sources of groundwater contamination. Effective risk assessment and remediation at these sites depends on the ability to accurately characterize the mass transfer of components between NAPL and groundwater (NAPL dissolution). The factors influencing dissolution of NAPLs have been examined in detail at the laboratory column scale. However, the fundamental dissolution behavior of NAPLs at the pore scale is not well understood. In addition, minimal research has been conducted to examine the impact of porous-medium heterogeneity on NAPL dissolution. The overall goal of the proposed research is to better understand the dissolution behavior of dense nonaqueous-phase, immiscible organic liquids (DNAPLs) in subsurface systems. The specific objectives developed to accomplish this goal are: (1) Investigate the dissolution dynamics of DNAPLs at the pore scale. (2) Develop and apply a lattice/network model to simulate dissolution dynamics at the pore scale. (3) Investigate the impact of porous-medium heterogeneity on DNAPL dissolution. (4) Investigate the impact of porous-medium heterogeneity on the efficacy of hydraulic-based source-zone remediation technologies. (5) Develop and evaluate advanced mathematical models capable of simulating the dissolution and transport of DNAPL constituents in heterogeneous porous media at multiple scales. The experiments are designed to investigate the impact of porous-medium properties on NAPL morphology and distribution, and the resultant effect on dissolution and mass-flux behavior. Experiments will be designed to allow investigation of complete dissolution and mass removal. The project involves both pore-scale and macro-scale investigations. In both cases, advanced imaging methods will be used to obtain direct, in-situ measurements of NAPL configuration and dissolution dynamics. This information will be integrated with measurements of mass flux data. Advanced mathematical modeling will be employed to help synthesize and interpret the results of the experiments. The project makes use of two National Laboratory user facilities that provide the opportunity to use advanced imaging techniques that would otherwise not be available.